Abstract Dynamic hyperinflation limits exercise duration in chronic obstructive pulmonary disease (COPD). To limit dynamic hyperinflation during exercise and thus improve exercise duration, we successfully developed a visual-auditory ventilation-feedback system. The system retrains patient's breathing pattern during exercise. The goal of the current proposal is to develop a user-friendly ventilation-feedback technique with a novel auditory feedback system. We reason that the proposed feedback system plus exercise training will be superior to exercise training alone while having the potential for an easier application into clinical practice. Hypotheses: The primary hypothesis is that the exercise duration of patients with moderate-to-severe COPD who successfully complete a 12-week program of breathing retraining plus exercise will be longer than that of patients who complete a 12-week program of treadmill exercise training alone. We also hypothesize that the primary predictor of improved exercise duration will be a reduction in dynamic hyperinflation and to a lesser extent, improvement in peripheral muscle function. Lastly, we hypothesize that dyspnea will be reduced in patients assigned to breathing retraining plus exercise when compared to exercise training alone. Methods: The proposed study is a randomized controlled clinical trial. 246 patients with moderate-to-severe COPD will be enrolled and randomized into breathing retraining plus exercise or exercise training alone. Both groups will receive 12-weeks of treadmill exercise training three times weekly. The breathing retraining plus exercise group will also receive auditory feedback to decrease respiratory rate and prolong exhalation. The goal of breathing retraining is to reduce exercise- induced dynamic hyperinflation. Follow-up testing will be completed at 6, 12, and 24 weeks. Testing will include a pulmonary function test, symptom-limited and constant workrate treadmill tests, six-minute walk, dyspnea measurements, testing of respiratory muscle strength and endurance, and quadriceps muscle endurance testing. Analysis: Measures of central tendency will be used to describe the study sample. A two-sample t-test ( = 0.05) will be used to analyze changes from baseline to 12-weeks between the breathing retraining plus exercise group and exercise training alone group. In data analysis, intention-to-treat principles will be used. Since several measures will be taken on each patient, mixed-models analysis will be used to compare changes over time between the two groups. Multiple regression analysis will be employed to determine the predictors of improved exercise performance.